Controlling the output voltage and thus the power of most of the high frequency (HF) amplifiers operating in a switched mode (e.g., class E amplifiers) can only be realized by controlling the input voltage of the HF amplifier. This input voltage corresponds to the output voltage of a direct current (DC) supply. Thus, the DC supply is included in the control. DC current supply units, in particular DC supply units operating in the switched mode, are often too slow for controlling the input voltage of a HF amplifier for plasma processes or laser excitation due to an internal clock and filter elements being present.
From U.S. Pat. No. 4,890,216 it is known to provide a linear series element immediately upstream of the HF amplifier. U.S. Pat. No. 4,890,216 recognizes that a great power loss and a high heat development exist along with this measure. Thus, a complete voltage control is not possible with a linear series element due to the high power loss. Therefore, U.S. Pat. No. 4,890,216 proposes the use of a fast, switched series element. Such a fast switched series element is, however, more complex in its construction than a conventional DC supply unit. In addition, a switched series element shows voltage source behavior and thus a low internal resistance. Therefore, undesired resonances may be excited easily, which are not sufficiently attenuated.